Rechargeable dry cell batteries are widely used as reliable sources of power to provide for portability of electrical tools, lighting equipment, electronic computer games and toys and the like. With the use of rechargeable batteries having extra long life, the costs of a rechargeable system are considerably less than using replaceable batteries when they become discharged. In many situations, there is little if any constraint with respect to the amount of time which may be allotted to recharging the smaller variety rechargeable dry cell batteries, particularly of the nickel cadmium type. Hence, heat generated during the charge cycle of the battery can be dissipated without seriously overheating and affecting the recharged battery's performance. However, there are circumstances where it is desirable to recharge the batteries as quickly as possible. Particularly with larger rechargeable batteries, this can result in the generation of excessive amounts of heat as the battery begins to reject portions of the charging current applied to the battery as it gets closer and closer to its maximum charge.
Heat generation particularly becomes a problem when the battery cells are enclosed in a sealed casing or the like which cannot readily dissipate the heat generated during the charging. In addition, heat generation in circumstances where a fast charge is require is a problem with large amp-hour rated batteries of the nickel cadmium type. In addition, electronic components, which may be used in the controlling of the recharging of such batteries, also generate heat which may not be readily dissipated during the charging cycle. All of these aspects then lend to heat generation either within the battery or in its immediate surroundings which can appreciably affect the maximum level of charge which can be placed on the battery. In accordance with recent studies, it has been determined that a battery, which appears to be fully charged at a temperature of 40.degree. C., is effectively only 80 percent charged when it cools down to room temperature for use. There have, therefore, been many attempts to control charging current during the battery charging cycle in a way which does not appreciably overheat the batteries. In many instances where time is not of the essence, slow charging phases can be adopted to preclude overheating of the battery during the final phases of battery charging. U.S. Pat. No. 3,626,270 discloses the use of charge/discharge cycles in the recharging of a battery. This approach provides for a rapid charging of rechargeable dry cell batteries. During charging intervals, charge current pulses are applied to the battery and subsequently by use of alternating current voltage, a charge/discharge current is applied to the battery. The system charging may be terminated upon sensing a maximum temperature for the battery and then switching to a trickle charge to maintain the battery charge.
U.S. Pat. No. 4,394,612 contemplates fast charging circuits and has the capability of charging batteries at a charging current of four times the "C" rating of the battery. Provision is made for sensing the transformer temperature. Upon attaining a predetermined temperature, fast charging of the batteries is terminated and interval trickle charge is applied to maintain the charge on the battery. In each of these situations, it is assumed that once the battery has attained a predetermined temperature or voltage regardless of the temperature, it is assumed that the battery is fully charged and hence is switched to trickle charging. However, it has been discovered that when the battery cools its extent of charge may be reduced by as much as twenty percent, compared to its apparent charge level when the battery is overheated.
U.S. Pat. No. 3,614,584 discloses pulse charging of rechargeable dry cell batteries. During the high rate of charge, current pulses are applied through a rectifying device for the AC charging current. Control is provided to reduce current flowing through the rectifying device as a selected terminal characteristic of the battery, such as voltage or temperature increases as charge progresses and stopping all current flow after attainment of either the sensed terminal battery voltage or terminal temperature. A silicon controlled rectifier is used in rectifying the AC charging current. A regenerative effect then takes place, due to the decreasing current causing the SCR to fire later in each cycle because of decreased gate sensitivity. A thermistor is used to sense battery temperature. A decrease in thermistor resistance, due to battery temperature increase, causes the SCR to fire later in each cycle. As a consequence, a point is reached where the SCR will no longer turn on and charging of the battery is terminated. When the battery voltage falls after termination of charge, the SCR will commence conducting on an infrequent basis to apply a trickle charge to the battery. Therefore, this device compensates for increased battery temperature and provides some cooling for the battery during the final stages of charging. However, the battery does not effectively cool during the latter stages of the charging, because charging current is maintained on the battery, albeit reduced, but still sufficient to cause heating of the battery due to tne battery's rejection of at least a portion of the charging current applied. It is, therefore, very difficult to achieve a reduced battery temperature during the final stages of charging, so that when the battery is removed from the charger and then is allowed to cool to ambient, the maximum possible charge is not provided in the battery.